The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus that can be retained on the observer's head or face.
As an example of conventional head- or face-mounted image display apparatus, an image display apparatus disclosed in Japanese Patent Application Unexamined Publication (KOKAI) NO. 3-101709 (1991) is known. FIG. 16(a) shows the entire optical system of the conventional image display apparatus, and FIG. 16(b) shows a part of an ocular optical system used in the image display apparatus. As illustrated in these figures, in the conventional image display apparatus, an image that is displayed by an image display device is transmitted as an aerial image by a relay optical system including a positive lens, and the aerial image is projected into an observer's eyeball as an enlarged image by an ocular optical system formed from a concave reflecting mirror.
U.S. Pat. No. 4,669,810 discloses another type of conventional image display apparatus. In this apparatus, as shown in FIG. 17, an image of a CRT is transmitted through a relay optical system to form an intermediate image, and the image is projected into an observer's eye by a combination of a reflection holographic element and a combiner having a hologram surface.
Japanese Patent Application Unexamined Publication (KOKAI) No. 67-214782 (1987) discloses another type of conventional image display apparatus. As shown in FIGS. 18(a) and 18(b), the conventional image display apparatus is designed to enable an image of an image display device to be directly observed as an enlarged image through an ocular lens.
U.S. Pat. No. 4,026,641 discloses another type of conventional image display apparatus. In the conventional image display apparatus, as shown in FIG. 19, an image of an image display device is transferred to a curved object surface by an image transfer device, and the image transferred to the object surface is projected in the air by a toric reflecting surface.
Reissued U.S. Pat. No. 27,356 discloses another type of conventional image display apparatus. As shown in FIG. 20, the apparatus is an ocular optical system designed to project an object surface onto an exit pupil by a semitransparent concave mirror and a semitransparent plane mirror.
In image display apparatuses of the type wherein an image of an image display device is relayed, as shown in FIGS. 16(a), 16(b) and 17, however, several lenses must be used as a relay optical system in addition to an ocular optical system, regardless of the type of ocular optical system. Consequently, the optical path length increases, and the optical system increases in both size and weight.
In a case where only the ocular optical system shown in FIG. 16(a) is used, as shown in FIG. 16(b), positive power resides in only the reflecting surface that has a concave surface directed toward the observer. Therefore, large negative field curvature is produced as shown by reference character P1 in the figure.
In a layout such as that shown in FIGS. 18(a) and 18(b), the amount to which the apparatus projects from the observer's face undesirably increases. Further, because an image display device and an illumination optical system are attached to the projecting portion of the apparatus, the apparatus becomes increasingly large in size and heavy in weight.
Because a head-mounted image display apparatus is fitted to the human body, particularly the head, if the amount to which the apparatus projects from the user's face is large, the distance from the supporting point on the head to the center of gravity of the apparatus is long. Consequently, the weight of the apparatus is imbalanced when the apparatus is fitted to the observer's head. Further, when the observer moves or turns with the apparatus fitted to his/her head, the apparatus may collide with something.
That is, it is important for a head-mounted image display apparatus to be small in size and light in weight. An essential factor in determining the size and weight of the apparatus is the arrangement of the optical system.
However, if an ordinary magnifier alone is used as an ocular optical system, exceedingly large aberrations are produced, and there is no device for correcting them. Even if spherical aberration can be corrected to a certain extent by forming the configuration of the concave surface of the magnifier into an aspherical surface, other aberrations such as coma and field curvature remain. Therefore, if the field angle for observation is increased, the image display apparatus becomes impractical. Alternatively, if a concave mirror alone is used as an ocular optical system, it is necessary to use not only ordinary optical elements (lens and mirror) but also a device for correcting field curvature by an image transfer device (fiber plate) having a surface which is curved in conformity to the field curvature produced, as shown in FIG. 19.
In a coaxial ocular optical system in which an object surface is projected onto an observer's pupil by using a semitransparent concave mirror and a semitransparent plane mirror, as shown in FIG. 20, because two semitransparent surfaces are used, the brightness of the image is reduced to as low a level as 1/16, even in the case of a theoretical value. Further, because field curvature that is produced by the semitransparent concave mirror is corrected by curving the object surface itself, it is difficult to use a flat display, e.g. an LCD (Liquid Crystal Display), as an image display device.